1. Field of the Invention
The invention relates to an antenna system for transmitting and receiving electromagnetic waves, in particular to a dual-polarized antenna.
2. Description of the Related Art
Horizontally or vertically polarized radiating element arrangements, for example in the form of dipoles arranged in the polarization plane, and slots arranged transversely with respect thereto or in the form of planar radiating elements, such as patch radiating elements, have been known for a long time. In the case of horizontally polarized radiating element arrangements, the dipoles are in this case arranged horizontally. Corresponding radiating element arrangements in the form of slots are in this case arranged vertically. Radiating element arrangements are likewise known which can be used for simultaneously transmitting and receiving waves with two orthogonal polarizations, and these are also referred to as dual-polarized antennas in the following text. Corresponding radiating element arrangements, for example comprising a plurality of elements in the form of dipoles, slots or planar radiating elements, are known from EP 0 685 909 A 1 or from the publication "Antennen" [Antennas], 2nd part, Bibliographical Institute, Mannheim/Vienna/Zurich, 1970, pages 47 to 50.
In order to improve directionality, these radiating element arrangements are normally arranged in front of a reflecting surface, the so-called reflector. Furthermore, it has been found to be advantageous for mobile radio applications for dual-polarized radiating element arrangements to be skewed, for example at +45.degree. or -45.degree., so that each system transmits linear polarization at +45.degree. or -45.degree., and the two systems are in turn orthogonal with respect to one another.
It has been found to be disadvantageous in the case of the various radiating element types that this alignment for +/-45.degree. polarization is in this case exact only in the main beam direction. Depending on the type of radiating element, the alignment of the polarization for a major angular deviation from the main beam direction may differ to a greater or lesser extent from the desired +45.degree. or -45.degree., and is thus dependent on the propagation direction. If, for example, the radiating element type is a dipole aligned at +45.degree. or -45.degree., then this is obviously comprehensible. Since only the projection of the dipole appears in the respective transmission direction, the polarization is, for example, virtually vertical at right angles to the main beam direction.
However, for +45.degree./-45.degree. dual-polarized antennas, it is desirable for the alignment of the linear polarization to be independent, that is to say at least largely independent, of the transmission direction. In the case of skewed polarization planes, which may be aligned, for example, at +45.degree. and -45.degree., this means that, even if the field strength vector is broken down vectorially into a horizontal and a vertical component, the polar diagrams of the vertical and horizontal individual components should have the same 3 dB beamwidth as the sum component.
Large horizontal 3 dB beamwidths of 60.degree.-120.degree. are preferably used for mobile radio applications; thus, in this case, the described effect of the dependency of the polarization alignment of the transmission direction in most radiating element types means that, in the horizontal polar diagrams for the vertical and horizontal individual components, the 3 dB beamwidth of the vertical component is larger than the 3 dB beamwidth of the horizontal component.
Thus, in the case of antennas with skewed polarization, in particular with the polarization plane aligned at +45.degree. and -45.degree., it has been found to be disadvantageous that it is impossible to use simple means to achieve 3 dB beamwidths of more than 85.degree.-90.degree. and, furthermore, with the means known to date, it is impossible to achieve virtually constant polarization alignment.
It is furthermore known that vertically arranged slot radiating elements, which are energized, for example, by means of a coaxial cable, a stripline or a triplate structure, may have a horizontally polarized radiation characteristic with a comparatively large horizontal 3 dB beamwidth.
In order to achieve defined 3 dB beamwidths, EP 0 527 417 A 1, for example, proposes the use of a plurality of offset slots, which are fed by means of a stripline, for beamforming. However, a disadvantage of this configuration is that the slots have a narrower 3 dB beamwidth than the individual radiating elements, that is to say they are directed to a greater extent at the start.
The prior publication U.S. Pat. No. 5,481,272, which represents the prior art, has disclosed a circularpolarized antenna system. The radiating element module comprises two dipoles arranged in cruciform shape with respect to one another, and aligned diagonally in a reflector box whose plan view is square. In other words, the reflector box base, which is arranged parallel to the dipole surfaces, forms the actual reflector plane which is provided all round with conductive boundary walls, aligned at right angles to the reflector plane. This prior publication thus describes a cruciform dipole arrangement for circular polarization.
DE VITO, G. et al.: Improved Dipole-Panel for Circular Polarization. In: IEEE Transactions on Broadcasting, Vol. BC-28, No. 2, June 1982, pages 65 to 72 describes a cruciform dipole arrangement, likewise for circular polarizations, in which the shape of the reflector is used to influence the polar diagram. In this case, the reflector plate likewise once again has a square shape in a plan view of the dipole cruciform which is aligned diagonally above it, and is surrounded by circumferential reflector walls which are aligned, for example, at an angle of 45.degree. to the reflector plane.
DE-GM 71 42 601 discloses a typical directional radiating element field for circular or electrical polarizations in order to form omnidirectional antennas.
Finally, the prior publication EP 0 730 319 A1 describes an antenna system having two dipole antennas which are arranged aligned vertically at a distance one above the other and are mounted in front of a reflector plate. The reflector plate is in this case provided with two side, external reflector sections or reflector vanes, which are angled forward about a bend edge running vertically and parallel to the dipoles. This is intended to change the antenna characteristic, in order to suppress transmission at the sides. To do this, the side reflector parts preferably use an edge angle which is between 45.degree. and 90.degree., that is to say with 90.degree. being at right angles to the reflector plane.
In addition, this antenna is also provided with two additional reflector rails which are fitted on the reflector surface and are located between the angled, side reflector sections and the dipoles that are seated such that they are aligned vertically, and which reflector rails have a longitudinal slot in the middle. The longitudinal slots are in this case located between the two vertical dipoles, and, in side view, are covered by the external reflector plate sections.